Helper T cells recognize a processed form of antigen (Ag) in association with class II histocompatibility molecules (MHC) expressed on the surface of Ag presenting cells (APC). It is important to understand how Ag is prepared for recognition by T cells because these cells play a central role in the regulation of immunity. Ag, taken up by endocytosis, passes through a poorly defined intracellular pathway. prior to expression on the surface of APC in a form that can be recognized by T cells. The long-term objective of this proposal is to delineate mechanisms responsible for regulation of Ag processing with emphasis on physiological conditions that affect the formation of Ag/MHC complexes. The first aim of the proposal is to employ biochemical techniques and functional assays to define features of the class II biosynthetic pathway that may influence Ag processing. The second aim focuses on the various mechanisms whereby intracellular pH gradients may regulate Ag/MHC complex formation. A cell-free constitution system is utilized in a third aim to identify and characterize components of subcellular organelles that regulate the class II binding and dissociation of Ag. It is expected that the proposed studies will provide novel information important for understanding how immune responses are initiated and regulated.